1. Field of the Invention
This invention relates to a method of manufacturing cold rolled steel sheets for extra deep drawing with excellent press formability and/or chemical conversion treating property.
2. Description of the Prior Art
In the manufacture of cold rolled steel sheets for use in the extra deep drawing, it has hitherto been adopted to add Ti to an extremely low carbon steel having a carbon content of 0.001-0.02% and perform the hot rolling at a temperature higher than the Ar.sub.3 transformation point as disclosed in Japanese Patent Application Publication No. 44-18,066. However, in such a method, as the carbon content becomes lower, the Ar.sub.3 transformation point rises, so that the hot finishing temperature (FT) must be set at not less than 880.degree. C. Thus, in order to secure this FT, the heating temperature of the cast slab must be raised from about 1,200.degree. C. used in the conventional low carbon steel (C.congruent.0.02-0.04%) to a high temperature of 1,250.degree.-1,350.degree. C., which has the following drawbacks:
(a) The energy consumed in the heating furnace becomes considerably and uneconomically larger; PA0 (b) Since the heating temperature becomes higher, there are caused the increase in the maintenance cost of the heating furnace, the reduction of the yield due to the increase in the amount of scale produced, the increase in the wear-out amount of the rolls, and the like; PA0 (c) In the case that the cast slab is directly subjected to a hot rolling without passing through a reheating furnace, the slab temmperature is apt to lower in the hot rolling, so that it is difficult to maintain the hot finishing temperature of not less than Ar.sub.3 transformation point and to obtain sheets of good quality.